The present invention relates to a process for preparing alkali metal and alkaline earth metal tricyanomethanides having a particularly high purity.
Ionic liquids containing alkali metal or alkaline earth metal tricyanomethanides (TCMs) are important raw materials and auxiliaries in the electronics industry for, inter alia, the production of rechargeable batteries. For most applications, it is necessary for the methanides used to be particularly pure, in particular halogen-free, in order to avoid corrosion problems and/or undesirable secondary reactions.
Various processes for preparing tricyanomethanides are known. The cyanidation of malononitrile (MN) was described for the first time by Schmidtmann in Chem. Ber. 1896, 29, 1168-1175. Here, MN is deprotonated by means of sodium ethoxide in ethanol and subsequently converted into sodium tricyanomethanide by stepwise addition of cyanogen chloride and subsequently crystallized from ether. In this process, sodium tricyanomethanide is isolated in a yield of about 70%.
In Chem. Ber. 1929, 62B, 153-163, Birckenbach et al. describe the cyanidation of MN by means of cyanogen bromide. Birckenbach et al. and Mayer et al. (Monatsh. Chem., 1969, 100, 462) have described the preparation of silver tricyanomethanide which is low in halogen by admixing crude alkali metal tricyanomethanide with silver nitrate, resulting in initial precipitation of silver chloride or silver bromide. Addition of further silver nitrate to the filtrate enables silver tricyanomethanide to be isolated. In addition, Mayer et al. have described the reaction of silver tricyanomethanide with cyanogen chloride at 100° C. for 40 hours to form tetracyanomethane which was sublimed and subsequently hydrolysed in sulphuric acid to form ammonium tricyanomethanide. Lithium tricyanomethanide was obtained by Mayer by addition of lithium chloride to an acetonitrile solution of tetracyanomethane at −96° C.
The preparation of high-purity potassium tricyanomethanide was disclosed for the first time by Hipps et al. (J. Phys. Chem. 1985, 89, 5459). In the process, potassium tricyanomethanide was dissolved in acetone, the solution was treated with activated carbon and the potassium tricyanomethanide was subsequently precipitated in diethyl ether. This procedure was repeated 10 times. To remove residual organic impurities, the potassium tricyanomethanide obtained was subsequently recrystallized twice from water. This gave a white crystalline powder which did not display a Raman fluorescence background on excitation with light of 5145 Å and was interpreted as highly pure.
WO-A-98/29389 discloses the cyanidation of MN by means of cyanogen bromide in the presence of 1,4-diazabicyclo[2.2.2]octane (DABCO®) in THF. In the process, DABCO® hydrochloride is crystallized out at −20° C. over a period of 28 hours. This gave 98% pure lithium tricyanomethanide.
A further process has been disclosed by Trofimenko et al. in J. Org. Chem. 1962, 27, 433, in which potassium tricyanomethanide was obtained by treating a dihalomalononitrile-potassium bromide complex with potassium cyanide.
In Bull. Soc. Chim. Fr. 1954, 948, Cox et al. have described a further process for preparing tricyanomethane at low temperature, in which bromomalononitrile was reacted with potassium cyanide.
Further processes for preparing tricyanomethanides comprising the reaction of deprotonated MN with phenyl cyanate have been disclosed by Grigat et al. in Chem. Ber. 1965, 98, 3777-3784, and Martin et al. in DD-A-48614. Yields of from 75 to 88% were obtained here.
A further method of purifying sodium tricyanomethanide by recrystallization from acetonitrile was described in 1987 by Bock et al. in Z. Naturforsch., 1987, 42b, 315, which gave sodium tricyanomethanide in a yield of 70% (without any indication of the purity).
None of the processes gives halogen-free products. The tricyanomethanides which can be obtained always have to be worked up further in more or less complicated purification steps. The success of this purification depends, inter alia, on the impurity profile, the crude product content and the consistency of the product.